Frozen Belt Tensioner

You have to loosen the tensioner pulley bolt to get tensioner moving. You can try to put some penetrating oil into back of the bolt.

Just get a good 6-point socket onto that bolt and long ratchet or small breaker bar to it. It will either open it or snap the bolt. If the bolt snaps in two you can still disassemble it. But probably the bolt will just open.

 

I remember that bolt too. Use a 6 point like Valde says and not a pos socket a name brand like Craftsman not harbor freight and put a pipe over the end of a breaker bar so its like 3 feet long. I use 3/4 emt piping from Lowes in the electrical section. Like $4. Sold in 10 foot sections you can cut it down to any length right there at the store with a hack saw. When the breaker bar is 3 feet long then your talking. Just put some pressure on it and wait...then some more...and then some more...go slow and eventually the bolt will walk off. Don't go full power right away or you'll snap the bolt off. Patience.
I keep a 3 footer in the trunk when I have to change a tire on the road. The damn tire stores put them on with impact wenches. Try getting them off with a regular socket wrench.

 

That bolt is a PITA. I would just buy a new tensioner and be done with it. I mangled that bolt on mine with I did my water pump R&R. Fortunately I took someone's adise on PC, most likely Ed's or Patrick's and bought a new tensioner at the time, in the event I knackered that bolt. And as advertised I destroyed the bolt. I did everything you've tried in the end I think I had to cut/grind off that bolt head to remove the tensioner assemble to replace it.

 

Coming from Honda, I was dismayed at the amount of rust on Prius bolts. I think their spec's are lax.

I'm getting cagy: anything I take off I'll lube before reinstall. And I'll go out of my way, to remove/lube/reinstall some. Or just coat them without removal, hopefully reduce moisture incursion, internal rust.